gistfile1.cs

            var _username = "Account.UserName";
            var _password = "Account.PassWord";
            var data = "wrap_name=" + _username + "&wrap_password=" + _password + "&wrap_scope=http://xxx.com/";
            var url = "https://www.starbucks.com/account/signin";

            var wc = new WebClient
            {
                //BaseAddress = "https://www.starbucks.com/account/signin"
            };
            var URI = new Uri(url);
            wc.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
            wc.UploadStringCompleted += new UploadStringCompletedEventHandler(wc__UploadStringCompleted);

            wc.UploadStringAsync(URI, "POST", data);